Apparatus for protecting electrical devices

ABSTRACT

Apparatus for protecting electrical devices, particularly permanent magnet motors and components for controlling the speed of such motors, such as solid state elements, against excessive current and excessive temperature. A permanent magnet motor utilized for driving an electric appliance, such as a blender, is controlled by a circuit which includes a silicon-controlled rectifier in series with the motor. The SCR is mounted in heat transfer relationship with one side of a heat sink composed of a material having good heat conductivity. A thermally-actuated switch, connected in series circuit with the motor and the SCR, is positioned in heat transfer relationship with the opposite side of the heat sink. The heat sink is constructed so that its thermal mass may be easily modified to make the temperature of the heat sink correspond generally to the temperature of the motor, at least in the range of temperatures where protection is to be provided. The thermally-actuated switch is responsive to the heat sink temperature to protect the electrical devices against excessive temperature of the motor or of the SCR, and also is responsive to current through the resistance of the switch to protect against excessive current surge.

' United States Patent Hamstra [54] APPARATUS FOR PROTECTING ELECTRICALDEVICES Inventor: David C. Hamstra, Clinton, Iowa Assignee: GeneralElectric Company Filed: Sept. 28, 1971 Appl. No.: 184,564

[5 6] References Cited UNITED STATES PATENTS 2/1967 Kitami ..3l7/4112/1969 Riley ..3l7/33 SC 8/1970 Routh et al. ..3l7 /4l X 4/1971Doversberger ..3l7/40 R Primary Examiner.lames D. TrammellAttorney-Joseph B. Forman et a1,

. [151 3,693,047 1 Sept.'19, 1972 [5 7] ABSTRACT Apparatus forprotecting electrical devices, particularly permanent magnet motors andcomponents for controlling the speed of such motors, such as solid stateelements, against excessive current and excessive temperature. Apermanent magnet motor utilized for driving an electric appliance, suchas a blender, is controlled by a circuit which includes asilicon-controlled rectifier in series with the motor. The SCR ismounted in heat transfer relationship with one side of a heat sinkcomposed of a material having good heat conductivity. Athermally-actuated switch, connected in series circuit with the motorand the SCR, is positioned in heat transfer relationship with theopposite side of the heat sink. The heat sink is constructed so that itsthermal mass may be easily modified to make the temperature of the heatsink correspond generally to the temperature of the motor, at least inthe range of temperatures where protection is to be provided. Thethermally-actuated switch is responsive to the heat sink temperature toprotect the electrical devices against excessive temperature of themotor or of the SCR, and also is responsive to current through theresistance of the switch to protect against excessive current surge.

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sum 2 or 4 INVENTOR DAVID C. HAMSTRA ATTORNEY PATENTEDSEPIQ m2 3.693.047SHEET 3 OF 4 I INVENTOR DAVID C. HAMSTRA j f i zfl,

ATTORNEY PATENTEDSEP 1 1912 3.693; 041

DAVID C HAMSTRA wfw ATTORNEY APPARATUS FOR PROTECTING ELECTRICAL DEVICESThis invention relates to apparatus for protecting electrical devices,and more particularly to apparatus for protecting the motor and solidstate control element of an electrical appliance against excessivetemperature and current.

DESCRIPTION OF THE PRIOR ART In connection with many electrical devices,for example electric appliances, it is necessary to include provisionfor protecting components, such as the motor and control, against damagefrom overheating or excessive current. For example, in the case ofelectric appliances such as blenders which include blades for choppingfood, the appliance may be potentially severely overloaded and damagedif solid food placed in the blender blocks the rotation of the blades,resulting in a locked rotor mode of the motor. This condition causes themotor to draw an excessive current, thereby generating excessive heat inthe motor. If this is continued for more than a very short time, themotor may be severely damaged. Moreover, the drawing of excess currentby the motor may also endanger elements of the control circuit of themotor. For example many such motors are controlled by a circuitincluding a solid state device, such as a silicon-controlled rectifier,and excessive current through the motor and hence through the SCR mayresult in damage to or destruction of the solid state device.

In appliance motors of the series alternating current type, it has beenpossible to place a thermally-actuated switch, such as any of a numberof switches including a heat-responsive bimetallic element, in directcontact with the motor, for example, in contact with the field winding,so as to be immediately responsive to excessive temperature thereof.

In the case of appliances using a permanent magnet direct current motorinstead of a series alternating current motor, the field windings arereplaced by a permanent magnet. The windings which are subject to damageupon overload and excessive heat are located on the rotor. It isimpracticable to place the thermallyactuated switch in contact with themoving rotor and, even if it were possible to do so, it would tend tocreate an imbalance in the rotor, particularly in the case of high speedmotors, such as blender motors. The problem of proper protection of suchmotors against overload and excessive current and heat has thereforebeen a difficult one.

Moreover, the solid state device employed in controlling the; supply ofdirect current to such motors to control the speed thereof is alsosubject to damage from excessive current resulting from an overloadcondition and it is necessary to provide suitable protection to preventsuch damage or alternatively to use a higher duty, more expensive solidstate device.

By the present invention, provision is made for accurately sensingexcessive temperatures of the motor and opening the circuit beforedamage can occur, without the necessity of positioning a temperatureresponsive device in direct contact with the motor itself. Provision ismade in a simple manner for obtaining a temperature response at aposition displaced from the motor but which, through the arrangement ofthis invention, ac-

curately tracks the motor temperature, or at least correspondsapproximately to the motor temperature in the range of temperatureswhere damage could occur.

Moreover, there is combined in this temperature responsive apparatus,provision for sensing the temperature of the solid state device andinsuring interruption of the circuit before that temperature can reachthe point of damaging the solid state device.

All of this accomplished in a simple and economical manner, and,moreover, in a manner which facilitates simple alteration of theprotective apparatus to provide for accurate response with respect toany electrical devices of this type which are to be protected againstexcessive temperature and current.

Accordingly, it is an object of this invention to provide an improvedapparatus for protecting a motor and a control element from damageresulting from excessive heat and current.

It is another object of this invention to provide improved apparatus forachieving such protection without necessity of positioning a temperatureresponsive element in contact with the motor itself.

It is a further object of this invention to provide protective apparatusof this type which includes provision for easily modifying the apparatusto insure response, at a location displaced from the motor, to atemperature which corresponds generally with that of the motor.

It is still a further object of this invention to provide in a singlecombined protective apparatus means for protecting both a motor and acontrol element therefor against both excessive temperature andexcessive current.

SUMMARY OF THE INVENTION In carrying out the objects of this inventionin one form thereof, there is provided apparatus for protecting electricdevices against excessive current and temperature. The invention isparticularly applicable for protecting permanent magnet direct currentmotors and solid state control elements therefor. The solid statecontrol element, which may be a silicon-controlled rectifier (SCR) isarranged in series with the motor. A heat sink, constructed of amaterial of good heat conductivity, is disposed in a position displacedfrom the motor, and the SCR is mounted in heat transfer relationshipwith one side of the heat sink. A thermally-actuated switch is mountedin heat transfer relationship with the opposite side of the heat sink soas to be responsive to heat transmitted to the heat sink from the SCR.The heat sink is constructed so that its thermal mass may be easilymodified to cause the temperature of the heat sink to correspondgenerally to the temperature of the motor, at least in the range oftemperatures where protection is to be provided. The thermally-actuatedswitch responds to the heat sink temperature to open the circuit of themotor and the SCR to protect these electrical devices against damagebecause of excessive current and temperature conditions.

DESCRIPTION OF THE DRAWINGS The above-mentioned and other features andobjects of this invention and the manner of attaining them will becomemore apparent and the invention itself will be best understood byreference to the following description of a preferred embodiment oftheinvention, taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a perspective view, partly broken away, of a blenderincorporating one form of this invention.

FIG. 2 is an enlarged view of a portion of FIG. 1 showing the protectiveapparatus in greater detail.

FIG. 3 is a diagram of the circuit for controlling the operation of amotor used to drive the blender.

FIGS. 4 and 5 are perspective views of opposite sides of the protectiveapparatus.

FIG. 6 is a graph comparing temperature variations.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGS. 1 and 2,there is shown a blender 10 which includes a base housing 11 and acontainer 12 mounted on the housing for receiving solids or liquids. Amotor 13 is positioned within the housing and is arranged in aconventional manner to engage and drive blades 14 positioned in thelower portion. of the container 12.

A blender is normally intended to be operated over a range of fairlyhigh speeds. In the particular embodiment shown a permanent magnetdirect current motor is employed and the blender includes a plurality ofpush button switches 15 in the base housing for setting the blender toany desired speed. These push buttons control the resistance in acontrol circuit, as will be described further in connection with thecircuit shown in FIG. 3. Also positioned within the base housing is acircuit board 16, which includes substantially all of the elements ofthe control circuit shown in FIG. 3, and the protective apparatus 17which forms the subject matter of this invention.

To provide direct current to the rotor windings of the motor 13 from aconventional household alternating current source, and to control thespeed of the motor, the control circuit shown in FIG. 3 is provided. Thecircuit includes terminals 18 for connection to any suitable alternatingcurrent source. A solid state device 19 is connected in series with themotor 13. In the particular form shown, the solid state deviceconstitutes ,a siliconcontrolled rectifier (SCR), connected in serieswith the motor 13 and controlled through a gate 20, to provide half-waverectified power to the motor. It will be understood that other suitablesolid state devices, such as transistors, triacs, etc., could beincorporated in the circuit in lieu of the SCR. Triggering of the SCR isaccomplished by developing a voltage across the gate-cathode junctionsufficient to trigger the SCR into conduction. A reference voltage isapplied to the gate 20 from a voltage divider comprising a plurality ofresistances 21-28. The reference voltage applied to the gate 20, isdetermined by closing any desired one of the push button switches 15, soas to connect the gate 20 to the selected point on the voltage dividerformed by the resistances 21-28. This reference voltage determines thespeed setting of the motor 13. In operation, if the back generatedvoltage of the motor 13 exceeds the reference voltage thus set, thegate-cathode junction remains reverse-biased and the SCR will .nottrigger. Conversely, if the back generated voltage of the motor fallsbelow that of the reference voltage, the gate is forward-biased and theSCR is turned on to provide power to the motor. In this manner, thespeed of the motor is maintained at any speed determined by the selectedone of the push button switches 15. A diode 29 prevents AC power fromthe terminals 18 from reverse biasing the gate 20 during the negativehalf cycle. Diode 30 serves as a commutating diode across the motor 13.A capacitor 31 is provided to smooth performance at low speeds, that is,to filter noise and provide delay.

In order to provide protection for the motor and the SCR, athermally-actuated switch, shown schematically at 32, is incorporated inseries with the SCR and the motor. This switch and its physicalarrangement in the protective apparatus is described in more detail inconnection with the description of the protective apparatus illustratedin FIGS. 4 and 5.

It is necessary to provide for an electrical appliance, such as ablender, suitable protection against excessive current and againstoverheating caused by an overload of the apparatus. For example, heavymaterial such as chunks of carrots placed in the container 12, may causethe blender blades 14 to jam, resulting in a locked rotor condition ofthe motor. The attendant excessive current through the motor and the SCRcould result in the burning out of either or both unless suitableprotection is provided. Also, a lesser overload continued for a shortperiod of time may also result in excessive heating of the motor and ofthe SCR and damage thereto. The protective apparatus of this inventionis arranged to provide protection under both of these conditions and toprovide such protection for both the motor and SCR.

In many electric appliances a series alternating current motor isemployed, and it is possible to provide a thermally responsive switch indirect association with the field windings of such a motor. Thus, theswitch can be arranged to be responsive directly to any excessivetemperature of the motor to interrupt the circuit before damage to themotor can occur. In the present apparatus, however, it is contemplatedthat a permanent magnet direct current motor will be employed, and insuch a motor the windings, which are the hot part of the motor and whichare subject to damage upon excessive temperature, are located on therotor. It is impracticable to place a conventional temperatureresponsive switch on the rotor in direct association with the windings,and moreover, such positioning would result in imbalance of the rotor.This problem is overcome by the present invention by providing theprotective apparatus which, while displaced from actual contact with themotor itself, still provides a temperature to be sensed which closelytracks that of the motor. As seen in FIGS. 1 and 2, the protectiveapparatus 17 is positioned, like the motor 13, within the base housing11, but is disposed adjacent the outer wall of the housing at a pointdisplaced from the motor. It will benoted that the positioning of thisprotective apparatus in the housing at the point shown makes theprotective apparatus subject to the same ambient conditions as the motoritself.

Referring now to FIGS. 4 and 5, the thermally-actuated switch,illustrated schematically at 32 in FIG. 3, is shown in structuralrepresentation. The switch is intended to be a conventional one,employing a bimetallic element having internal resistance .so that thebimetallic element is heated and actuated under excessive currentconditions. Since such conventional switches are well-known, and sincethe details of internal structure are unnecessary for an understandingof this invention, such details of the internal structure are not shown.The switch includes a push button 33 for manually resetting the switch,should the switch be actuated under overload conditions. The switch ismounted in direct heat transfer relationship with one face of a heatsink 34. More specifically, the switch is mounted on the heat sink bymeans of a plurality of rivets 35. The SCR, which is shown schematicallyat 19 in FIG. 3, and physically at 19in FIG. 4, is positioned in directheat transfer relationship with the opposite face of the heat sink 34and approximately in the center of that face. The heat sink 34 is madeof a material of a high heat conductivity, so as to conduct heat rapidlyfrom the SCR. In the form shown the heat sink is made of copper platedwith tin. The temperature of the heat sink is sensed effectively by thethermally-actuated switch 32 because of its direct heat transferrelationship therewith.

Structurally the heat sink includes a generally rectangular flat plate36, and a flange 37 extending generally at right angles to the plate 36.The flange 37 has an opening 38 therein, which is intended to receive aselftapping screw for mounting the protective apparatus on a stud formedin the base housing 11.

It has been mentioned earlier in this specification that with apermanent magnet motor it is impracticable to mount a thermally-actuatedswitch in association with the hot part of the motor since the windingswhich are subject to damage upon overloading conditions are on therotating portion of the motor. By this invention this problem isovercome by causing the heat sink, which is displaced from the motor, totrack the temperature of the motor, or in any event to track thetemperature of the motor in the range of temperatures where protectionis to be provided. Moreover, this is accomplished in a simple manner sothat any necessary alteration to adapt the protective apparatus for usewith a particular type of motor may be accomplished quickly and easily.

In the particular form of the invention shown, the

heat sink receives heat from the SCR and the SCR,

being in series with the motor 13 is subjected to the same current asthe motor. If the current through the motor increases, for example,under overload conditions, there is a similar increase in the currentthrough the SCR. The resultant increased temperature causes greater heatto be transferred to the heat sink. It will be apparent, therefore, thatthe temperature of the heat sink will be related to the temperature ofthe motor. The actual increase in temperature of the heat sink for agiven current through the motor and the SCR, is dependent upon severalpotentially variable factors, including its thermal mass, surface area,material and configuration. The materialv is chosen as one of highconductivity so as to conduct heat rapidly from the SCR, and theconfiguration is also chosen to promote quick heat transfer from theSCR. The amount of surface area will, of course, affect the rate of heatdissipation from the heat sink to the surrounding area. The predominantfactor, however, in determining the temperature of the heat sink underany particular conditions is the thermal mass thereof.

In accordance with this invention, the thermal mass of the heat sink isselected so that the temperature of the heat sink closely tracks thetemperature of the motor. It will be apparent that the characteristicsof the heat sink must be varied for use with different types and sizesof motors, since the operating characteristics and temperatures ofmotors of different types and sizes will be different. In accordancewith the present invention, the heat sink is constructed so that it maybe modified, if necessary, in a very simple manner to cause the heatsink to track the motor temperature closely enough to provide thedesired protection. It is actually only necessary that the temperatureof the heat sink track, within an acceptable degree of tolerance, themotor temperature in the range of temperatures where the protection isdesired; that is, the temperature of the heat sink may, withoutsignificant effect on the protection provided, depart materially fromthe motor temperature in the range of temperatures below the maximumsafely permitted so long as the temperature of the heat sink closelyapproximates that of the motor when the region of maximum temperature tobe permitted is reached.

In carrying out the invention, the protective apparatus is assembled asshown in FIGS. 4 and 5 and is connected, under conditions to beencountered in operation, to a motor of the particular type to beemployed. The temperature of the motor is then measured and graphed, andsimultaneously the temperature of the heat sink is measured and graphed.Graphs illustrative of this situation, one for the motor and one for theheat sink employed in the preferred embodiment, are shown in FIG. 6. Itis contemplated that the maximum operating temperature of the motor andthe SCR be approximately 105 C. The protective apparatus is thereforedesigned to interrupt the normal line power at C. so as to provide forvariations which may occur in the manufacture of motors and SCRs.

It can be seen from FIG. 6 that the temperatures of the heat sinkemployed in this embodiment approximately track the temperatures of themotor so as to provide the needed protection. If desired, furthermodification of the heat sink could be made, in accordance with thisinvention, to cause the heat sink temperatures to track the motortemperature more closely, but it has been found that the protectiveapparatus performs satisfactorily under the conditions illustrated inFIG. 6. Moreover, it will be noted that the heat sink temperatures shownin FIG. 6 exceed those of the motor slightly, insuring that the heatsink will cause the thermally-actuated switch to open before thetemperature of the motor reaches a point where damage to the motor couldoccur.

If it were found, however, in an initial design of the protectiveapparatus for use with a particular electrical device that heat sinktemperatures were, for example, sufficiently below the temperatures ofthe motor that the protective apparatus would not provide properprotection for the motor against overload, it is merely necessary totrim the plate 36 of the heat sink 34 to remove a portion thereof, so asto reduce the thermal mass thereof, and thereby cause the heat sinktemperature to be higher for a given set of conditions.

It will be apparent that once the heat sink has been modified in thismanner to cause its temperature to track approximately that of a motorof particular type and characteristics, it will then be necessary onlyto duplicate the material, configuration, size, etc. of the heat sink somodified and the protective apparatus will perform satisfactorily withmotors of this type and characteristics used in mass production of theblenders. Thus, by a simple and easily accomplished modification thenecessary characteristics of the heat sink are achieved and massproduction of the protective apparatus is facilitated.

Thus, while it is not possible to recite structural details of the heatsink of general application since the characteristics will necessarilyvary for use with different types and sizes of motors having differentcharacteristics, provision is made in accordance with this inventionwhereby the heat sink may be modified empirically to adapt it for usewith any particular type and size of motor. The structure which formsthe subject matter of this invention permits this modification to beaccomplished in a very simple and expeditious manner.

In carrying out this invention in one specific form thereof, a one-halfhorsepower permanent magnet direct current motor was employed to drivethe blender. This motor included a permanent magnet field consisting oftwo poles of ferrite magnetic material enclosed in an outer steelcasing. The armature had a lamination stack 3 inches in diameter andtwenty-seven thirty-seconds inches long, and including 16 slots. Thewinding included 22 turns per coil of No. 22AWG wire with two coils perslot. A commutator of 32 bars was employed to relieve commutating arcingdue to the high current caused by operation on half-wave rectifiedalternating current line voltage. The direct current resistance of themotor was approximately 2 ohms and the inductance in the order of tomillihenries. The motor was designed to run at 7,100 rpm no load.

The SCR employed was of a conventional type comprising a reverseblocking triode thyristor with a repetitive peak voltage of 400 volts,an RMS on-state current of 8 amperes, a peak one-cycle surge on-statecurrent of 80 amperes, and a maximum operating temperature of 105 C. Aspecific SCR suitable for this purpose is identified by General Electrictype No. C122D.

The heat sink employed with this motor and SCR was made of copperapproximately 0.032 inches thick, plated with tin. The overalldimensions of the generally rectangular plate 36 were 1.25 inches long,L06 inches wide at the left end, and 1.05 inches at the right end (asviewed in FIG. 4). The chamfered edge 39 constituted the hypotenuse of atriangle having sides of approximately 0.4 inches and 0.13 inches. Theflange 37 had dimensions of 0.73 inch X 0.55 inch.

The thermally-actuated switch 32 included a bimetallic element and amanual reset button. The bimetallic element effected tripping of theswitch at approximately 95 C. Any conventional thermally-actuated switchtripping at the desired temperature may be employed. In the particularapparatus described, a General Electric Type No. 3ARG8S2-Z was employed.The motor in normal operation drew 3 to 4 amperes. Operating in thiscurrent range, the motor would normally run 10 minutes or more beforereaching the temperatures shown in FIG. 6. The switch was designed totrip in 9 seconds under an overload current of 18 amperes.

It will be apparent by this invention that an easily constructed andassembled protective apparatus is provided in which a thermallyactuatedswitch displaced from the motor is caused to respond to a temperatureaccurately simulating that of the motor through the use of a heat sinkassembled in heat conductive relationship with the switch and with anSCR utilized in controlling the speed of the motor.

It will be understood that, while the principles of this invention havebeen described in connection with a specific embodiment thereof, thisembodiment is merely illustrative of one form of the invention and theinvention is not limited to the specific embodiment. For example, theprotective apparatus is usable with many types of motor and variationsin the size, configuration, and other characteristics of the heat sinkmay be made, in accordance with the guidelines set forth in thespecification, without departing from the spirit and scope of thisinvention. It is intended, therefore, by the appended claims to coverall modifications which fall within the spirit and scope of thisinvention.

What is new and desired to be secured by Letters Patent of the UnitedStates is:

1. In an electrical appliance including a motor fordriving the applianceand a solid state device for controlling the speed of the motor,apparatus for protecting the motor and the solid state device againstoverheating and overload comprising:

a. a heat sink b. a thermally-actuated switch in series with the motorand the solid state device and mounted in heat exchange relationshipwith said heat sink c. said solid state device being mounted in heatexchange relationship with said heat sink whereby heat developed in saidsolid state device is transmitted to said'switch d. said heat sink beingselected of such thermal mass that its temperature correspondsapproximately to the temperature of said motor in the region of maximumsafe temperature for operation of said motor and said solid statedevice.

2. The apparatus of claim 1 wherein said solid state device is asilicon-controlled rectifier.

3. The apparatus of claim 1 wherein said heat sink is formed of amaterial of high heat conductivity for conducting heat rapidly from saidsolid state device.

4. The apparatus of claim 1 wherein said heat sink comprises a flatplate and wherein said switch is mounted on one face of said plate andsaid solid state device is mounted on the other face of said plate.

5. The apparatus of claim 4 wherein said heat sink further includes aflange extending from said plate and said flange has an opening thereinfor facilitating the mounting of said apparatus.

6. The apparatus of claim 4 wherein said plate is of generallyrectangular configuration and said solid state device is mountedapproximately in the center of said other face.

7. In an electrical appliance including a housing and a motor in thehousing for driving the appliance and further including a solid statedevice for controlling the speed of the motor, apparatus for protectingthe motor and the solid state device against overheating and overloadcomprising:

a. a heat sink b. a thermally-actuated switch in series with the motorand the solid state device and mounted on said heat sink in heatexchange relationship therewith c. said solid state device being mountedon said heat sink in heat exchange relationship therewith whereby heatdeveloped in said solid state device is transmitted to said switch d.said apparatus being positioned in said housing so as to be subject tothe same ambient conditions as the motor e. said heat sink including aflange having an opening therein for facilitating the mounting of saidapparatus on said housing f. said heat sink being selected of suchthermal mass that its temperature corresponds approximately to thetemperature of said motor in the region of maximum safe temperature foroperation of said motor and said solid state device.

8. In an electrical appliance,

a. a permanent magnet direct current motor for driv- 10 ing theappliance b. a solid state device in series with said motor forcontrolling the speed of the motor, and c. apparatus for protecting themotor and the solid state device against overheating and overloadcomprising:

1. a heat sink 2. a thermally-actuated switch in series with the motorand the solid state device and mounted in heat exchange relationshipwith said heat sink 3. said solid state device being mounted in heatexchange relationship with said heat sink whereby heat developed in saidsolid state device is transmitted to said switch 4. said heat sink beingselected of such thermal mass that its temperature correspondsapproximately to the temperature of said motor in the region of maximumsafe temperature for operation of said motor and said solid statedevice.

1. In an electrical appliance including a motor for driving theappliance and a solid state device for controlling the speed of themotor, apparatus for protecting the motor and the solid state deviceagainst overheating and overload comprising: a. a heat sink b. athermally-actuated switch in series with the motor and the solid statedevice and mounted in heat exchange relationship with said heat sink c.said solid state device being mounted in heat exchange relationship withsaid heat sink whereby heat developed in said solid state device istransmitted to said switch d. said heat sink being selected of suchthermal mass that its temperature corresponds approximately to thetemperature of said motor in the region of maximum safe temperature foroperation of said motor and said solid state device.
 2. The apparatus ofclaim 1 wherein said solid state device is a silicon-controlledrectifier.
 2. a thermally-actuated switch in series with the motor andthe solid state device and mounted in heat exchange relationship withsaid heat sink
 3. said solid state device being mounted in heat exchangerelationship with said heat sink whereby heat developed in said solidstate device is transmitted to said switch
 3. The apparatus of claim 1wherein said heat sink is formed of a material of high heat conductivityfor conducting heat rapidly from said solid state device.
 4. Theapparatus of claim 1 wherein said heat sink comprises a flat plate andwherein said switch is mounted on one face of said plate and said solidstate device is mounted on the other face of said plate.
 4. said heatsink being selected of such thermal mass that its temperaturecorresponds approximately to the temperature of said motor in the regionof maximum safe temperature for operation of said motor and said solidstate device.
 5. The apparatus of claim 4 wherein said heat sink furtherincludes A flange extending from said plate and said flange has anopening therein for facilitating the mounting of said apparatus.
 6. Theapparatus of claim 4 wherein said plate is of generally rectangularconfiguration and said solid state device is mounted approximately inthe center of said other face.
 7. In an electrical appliance including ahousing and a motor in the housing for driving the appliance and furtherincluding a solid state device for controlling the speed of the motor,apparatus for protecting the motor and the solid state device againstoverheating and overload comprising: a. a heat sink b. athermally-actuated switch in series with the motor and the solid statedevice and mounted on said heat sink in heat exchange relationshiptherewith c. said solid state device being mounted on said heat sink inheat exchange relationship therewith whereby heat developed in saidsolid state device is transmitted to said switch d. said apparatus beingpositioned in said housing so as to be subject to the same ambientconditions as the motor e. said heat sink including a flange having anopening therein for facilitating the mounting of said apparatus on saidhousing f. said heat sink being selected of such thermal mass that itstemperature corresponds approximately to the temperature of said motorin the region of maximum safe temperature for operation of said motorand said solid state device.
 8. In an electrical appliance, a. apermanent magnet direct current motor for driving the appliance b. asolid state device in series with said motor for controlling the speedof the motor, and c. apparatus for protecting the motor and the solidstate device against overheating and overload comprising: